The reaction of isocyanate and hydroxy compounds to form urethanes is the basis for the production of polyurethanes. Metal compounds (e.g., tin, zinc and bismuth compounds) and tertiary amines have been known to catalyze the reaction of isocyanate and hydroxyl groups to form urethane. See, Proceedings of Water Borne and High Solids Coatings Symposium, Feb. 25-27, 1987, New Orleans, at Page 460. Compounds useful for the isocyanate-hydroxy reaction are also referred to as urethane catalysts. At present, the commercially available catalysts used in this reaction are organotin compounds (e.g., dibutyltin dilaurate and dibutyltin diacetate), zinc carboxylates, bismuth carboxylates, organomercury compounds and tertiary amines.
There are several problems with these commercially available catalysts. When they are used in the process for polyurethane coatings, the cure of the coatings under high humidity or at low temperature conditions is not satisfactory. They catalyze the undesirable side reaction of isocyanate with water to form amines and carbon dioxide. The carbon dioxide may cause blisters in the coating and the amines react with isocyanates resulting in low gloss coatings. Moreover, the cure rate at low temperatures is too slow. The commercially available catalysts also catalyze the degradation of the resulting polymer product. Furthermore, several of the commercially available urethane catalysts, particularly those containing heavy metals and tertiary amines, are highly toxic and are environmentally objectionable.
The testing of zirconium acetylacetonate and zirconium tetra-3-cyanopentanedionate, as catalysts for the isocyanate-hydroxy reaction have been described in GB Patents 908949, 890,280 and 869988. Subsequent testing by others, however, has shown that zirconium acetylacetonate is a poor catalyst for the urethane reaction. B. D. Nahlovsky and G. A. Zimmerman, Int. Jahrestag. Fraunhofer--Inst. Treib-Explosivst., 18th (Technol. Energ. Mater.), 39:1-12, reported that the catalytic efficiency of zirconium acetylacetonate for the isocyanate-hydrox reaction to form urethane is low. The solubility of zirconium acetylacetonate and zirconium tetra-3-cyanopentanedionate in solvents commonly used in the production of coatings is poor. Examples of such solvents include esters ketones, glycolesters and aromatic hydrocarbons, such as: butyl acetate, methyl iso-amyl ketone, 2-methoxy propylacetate, xylene and toluene. Because of the low catalytic efficiency and the poor solvent solubility, the use of these compounds as catalysts in processes involving urethane or polyurethanes have been limited.
Further testing using zirconium acetylacetonate in our laboratory has shown that zirconium compounds disclosed in the prior art, will only catalyze the isocyanate-hydroxy reaction when carried out in a closed system, i.e., in a closed pot. This is impractical for many of the polyurethane applications. The zirconium diketonates of the prior art failed as catalysts when the reaction is carried out in the open atmosphere, unless there is present a large excess of the corresponding diketone. For zirconium acetylacetonate, the presence of over 1000 to 1 mole ratio of 2,4-pentanedione to zirconium acetylacetonate is required. However, 2,4-pentanedione and other similar diketones are volatile solvents which, when used in an open vessel, pollute the air, and pose both an environmental and a fire hazard. In addition, the presence of the free diketone causes discoloration of the catalyst, resulting in an undesirable, discolored product.
The objective of this invention is to develop catalysts with high catalytic efficiency for the isocyanate-hydroxy reaction to form urethane and/or polyurethane.
A second objective of the present invention is to develop catalysts which provide improved cure at a lower temperature and is less sensitive to the presence of water.
A further objective of the present invention is to develop metal diketonates as catalysts which would not be deactivated when the reaction is exposed to the atmosphere nor require an excess of free diketone.
Another objective of the present invention is to provide catalysts for the isocyanate-hydroxy reaction which would not catalyze the undesired side reaction of water with isocyanates or the undesired degradation of the polyurethane.